Effects Of Simulated Precipitation Change,and Nitrogen Deposition On Soil Microbial Community Structure In A Forest In Subtropical-Warm Temperate Climate Transition Zone

Soil microorganisms,as decomposers in ecosystems are involved in a series of biogeochemical processes.Soil microorganisms rapidly respond to environmental changes.Climate models predict that precipitation patterns,including amount,will change at global and regional scales.As a result of human activities,a large amount of active nitrogen?N?enters atmosphere and terrestrial ecosystem via atmospheric N deposition.Global N deposition rate shows a continuously increasing trend.China has been one of the three regions with the highest atmospheric N deposition rate in the world.Forest ecosystems play an important role in terrestrial carbon cycling.The Transition Zone Forest is sensitive and fragile,thus this region can respond quickly to environmental changes.However,effects of precipitation change and N deposition on soil microbial and underlying mechanisms in forest ecosystems in the climate Transition Zone are poorly understood.A manipulative experiment was set up in a deciduous broad-leaved forest ecosystem of the Transition Zone between subtropical and warm temperate regions in Central China.The manipulative experiment included six treatments(control,increased 50% precipitation,decreased 50% precipitation,N addition 50 kg N ha^-1 year^-1,increased precipitation and N addition,as well as decreased precipitation and N addition).The effects of precipitation change and N addition on soil microbial biomass and community structure were studied by phospholipid fatty acids?PLFAs?.The experimental results showed that increased precipitation enhanced soil moisture by 1.09 V/V%,but had no effect on soil temperature,pH,nitrate N,ammonium N,total organic C,total N,the ratio of total C and N,and fine root biomass.Decreased precipitation reduced soil moisture and ammonium N by 1.92 V/V% and 1.68%,elevated soil nitrate N,total organic C,and the ratio of total C and N by 23.66%,14.38% and 1.31?absolute change?,but did not affect soil temperature,pH,total N,and fine root biomass.Nitrogen addition decreased soil pH 0.04?absolute change?,improved nitrate N?+26.80%?,ammonium N?+21.54%?,and total N?+6.91%?.Nitrogen addition had no effect on soil temperature,moisture,total organic C,the ratio of total C and N,and fine root biomass.Increased precipitation had no effect on soil microbial biomass C and N.Decreased precipitation did not affect soil microbial biomass C but reduced microbial biomass N by 10.00%.Nitrogen addition decreased soil microbial biomass C and N by 20.75% and 10.35%,respectively.Increased precipitation and N addition had antagonism action to soil microbial biomass C.Soil microbial community structure showed no response to increased precipitation.Decreased precipitation suppressed the amount of soil microbial total PLFAs,Gram-negative bacteria PLFAs,Gram-positive bacteria PLFAs,and bacteria PLFAs by 14.58%,18.53%,17.92%,and 16.63%,respectively.By contrast,the amount of soil fungi PLFAs and the soil fungi and bacteria rate were enhanced by 27.21% and 0.07?absolute change?,respectively,under decreased precipitation,whereas PLFAs of actinomyces,arbuscular mycorrhizal fungi,and the Gram-positive and Gramnegative bacteria rate remained unchanged.Nitrogen addition decreased the amount of soil microbial total PLFAs?-14.26%?,Gram-negative bacteria PLFAs?-13.85%?,Gram-positive bacteria PLFAs?-20.31%?,arbuscular mycorrhizal fungi PLFAs?-19.79%?,bacteria PLFAs?-13.35%?,fungi PLFAs?-29.25%?,the Gram-positive and Gram-negative bacteria rate?-0.08 absolute change?,and the fungi and bacteria rate?-0.03 absolute change?.Actinomyces had no response to N addition.Increased precipitation and N addition had antagonistic interaction on Gram-positive bacteria PLFAs and the Gram-positive and Gram-negative bacteria rate.This study showed that increased precipitation does not influence the biomass and structure of soil microbial community.Decreased precipitation and N addition suppressed the availability and diffusion of substrates and the activity of soil microbe and enzyme by reducing soil moisture and pH,respectively.Decreased precipitation and N addition reduced soil microbial biomass and PLFAs,which lead to the accumulation of soil organic C.Increased precipitation weakened the negative effects on soil microbial biomass C,Gram-positive bacteria PLFAs and the Gram-positive and Gram-negative bacteria rate by enhancing the availability of nutrient.The results and findings in this thesis are critical for better understanding the response of forest ecosystem C pool and soil microbial community to global change.